1. Field of the Invention
The present invention relates generally to echo cancellation in communication networks. More particularly, the present invention relates to canceling secondary echoes in communication networks.
2. Background Art
Subscribers use speech quality as the benchmark for assessing the overall quality of a telephone network. A key technology to provide a high quality speech is echo cancellation. Echo canceller performance in a telephone network, either a TDM or packet telephony network, has a substantial impact on the overall voice quality. An effective removal of hybrid echo inherent in telephone networks is a key to maintaining and improving perceived voice quality during a call.
Echoes occur in telephone networks due to impedance mismatches of network elements and acoustical coupling within telephone handsets. Hybrid echo is the primary source of echo generated from the public-switched telephone network (PSTN). As shown in FIG. 1, hybrid echo 127 is created by a hybrid (not shown) within SLIC (Subscriber Line Interface Circuit) 125 of PSTN 120, which converts a four-wire physical interface into a two-wire physical interface for communication with telephone 110 having a two-wire physical interface. SLIC 125 includes a switched hybrid circuit operable during a POTS (Plain Old Telephone Service or System) mode when the associated subscriber is using a POTS station and in an ISDN (Integrated Services Digital Network) mode when the subscriber is using an ISDN station. The hybrid circuit includes a first amplifier circuit for coupling a signal at the two-wire port thereof to its four-wire transmit port, and a second amplifier circuit for coupling a signal at the four-wire receive port of the hybrid to the two-wire port. The hybrid reflects electrical energy back to the speaker from the four-wire physical interface.
As shown in FIG. 1, in conventional telephone network 100, VoIP (Voice over Internet Protocol) device 140 at Central Office (CO) 140 includes echo canceller 140 that is typically positioned between SLIC 125 and packet network 150. Generally speaking, echo cancellation process involves two steps. First, as the call is set up, echo canceller 145 employs a digital adaptive filter to adapt to the far-end signal and create a model based on the far-end signal before passing through the hybrid within SLIC 125. After the local-end signal, including near-end signal and/or echo signal, passes through the hybrid, echo canceller 145 subtracts the far-end model from the local-end signal to cancel hybrid echo and generate an error signal. Although this echo cancellation process removes a substantial amount of the echo, non-linear components of the echo may still remain. To cancel non-linear components of the echo, the second step of the echo cancellation process utilizes a non-linear processor (NLP) to eliminate the remaining or residual echo by attenuating the signal below the noise floor.
Today, conventional echo cancellers may be SPARSE echo cancellers, which employ adaptive filter algorithms with a dynamically positioned window to cover a desired echo tail length, such as a sliding window, e.g. a 24 ms window, covering an echo path delay, e.g. a 128 ms delay. To properly cancel the echo, the echo canceller must determine a pure delay or a bulk delay, which is indicative of the location of the echo signal segment or window within the 128 ms echo path delay. Non-SPARSE echo cancellers, on the other hand, utilize a 128 ms window, which covers the entire echo path delay, and do not need to determine the bulk delay. However, non-SPARSE echo cancellers are not as desirable as SPARSE echo cancellers due to high complexity and high cost, and further due to slow convergence because of the long echo tail and the higher number of tabs required for the adaptive filter.
Although conventional sparse echo cancellers aim to cancel the above-described primary echo caused by the hybrid within SLIC 125 of PSTN 120, conventional sparse echo cancellers fail to properly address and cancel additional secondary echoes in the network, which are not originated by the hybrid within SLIC 125 or by other equipment in the vicinity of SLIC 125. Conventional sparse echo cancellers operate based on a false assumption that the line echo occurs only at the hybrid in PSTN 120 due to the conversion a four-wire physical interface into a two-wire physical interface for communication with telephone 110, or in the vicinity of the hybrid in PSTN 120.
Since sparse echo cancellers need to dynamically determine the echo bulk delay, due to signal conditions, such as echo to noise ratio and talker loudness differences, it is possible to make wrong echo bulk delay decisions, which would result in a lack of echo control capability. This problem is even more pronounced when there is multiple echo sources with different bulk delays and different echo energy. Even by using multiple sparse active filter windows, a secondary echo location may not be correctly determined.
Accordingly, there is a need in the art for echo cancellers that cancel secondary echoes, as well as the primary echo, efficiently and effectively, and with a low level of complexity and memory consumption.